Fused silica or quartz articles are well-known for certain physical characteristics which render them unique among glasses. For example, such glasses demonstrate excellent refractoriness, enabling their use at high temperatures; they exhibit chemical inertness, especially to acidic environments; and they possess a low coefficient of thermal expansion, i.e., about 5-10 .times. 10.sup.-7 /.degree.C. over the range of 0.degree.-300.degree. C. Such articles are presently formed by the fusion of slip cast structures, the fusion of structures deposited via flame hydrolysis techniques, or by melting at very elevated temperatures, e.g., 2000.degree. C. and higher. The geometry and dimensions of shapes produced by those methods are somewhat limited and the physical properties vary according to the source of raw materials. Vitreous silica of very high purity is of great utility in the manufacture of waveguides.
High purity, vitreous silica has been prepared through the flame hydrolysis of such materials as silicon tetrachloride. The basic disclosure in that field is U.S. Pat. No. 2,272,342. However, because of the very nature of that production technique, the shape of the articles that can readily be formed is severely limited and the cost of manufacture is very high. Therefore, the capability of preparing fused silica articles of very high purity at relatively low cost and in essentially unlimited shapes would obviously be most attractive.
U.S. Pat. No. 3,678,144 describes a method for producing porous and non-porous silica-containing articles through the gelation of various aqueous silicate solutions. In carrying out the process, an aqueous solution having a pH between 10-15 and containing about 1-12 moles of SiO.sub.2 per liter in solution was prepared from such aqueous silicate solutions as alkali metal silicates, colloidal silica, quaternary ammonium silicate, and mixtures thereof. A metal oxide was then added to that solution in such a form that it could be dissolved therein without reducing the pH thereof below 10. Thereafter, an organic reagent selected from the group consisting of formaldehyde, paraformaldehyde, formamide, glyoxal, methyl formate, methyl acetate, ethyl formate, ethyl acetate, and mixtures thereof was blended into the solution to reduce the pH of the solution and thereby cause polymerization of the silica to a coherent, porous mass, the admixed metal oxide becoming part of the silica network. In an optional embodiment, free alkali metal oxide could be removed via a leaching step in weak acids. Finally, where desired, the porous body could be consolidated to a solid glass through firing at elevated temperatures.
Other disclosures relating to the polymerization of aqueous silicate solutions utilizing the above-mentioned organic gelling agents can be found in U.S. Pat. Nos. 3,782,982 and 3,827,893.
Although porous articles could be secured by following the methods described in those patents, the total porosity and the diameter of the pores developed could be quite variable. This failing effectively proscribed the utility of those bodies in such applications as catalyst supports for air pollution control, filtration devices for gases and/or liquids, acoustical materials, and chromatographic supports.
U.S. application Ser. No. 440,693, filed Feb. 8, 1974 in the names of Blaszyk, Shoup, and Wein, now abandoned described means for preparing porous particulate and monolithic silica-containing materials having a broad range of pore sizes, but with a narrow distribution of pore sizes within a particular range. Thus, porous bodies could be produced wherein the pores varied in size between about 100A-10,000A (1 micron) and the mean pore size distribution comprised at least 80% being no greater than .+-.30% and, commonly, less than .+-.10% from the average diameter. Such bodies were prepared utilizing specifically-defined compositional combinations of aqueous lithium polysilicate, sodium silicate, potassium silicate, quaternary ammonium silicate, and colloidal silica with the above-noted organic gelation agents. In essence, the foundation of that invention was the determination of the required combination of aqueous silicates which, when polymerized by the organic gelling agent, yielded the desired porous structure. Leaching with weakly acidic leachants to remove free alkali metal oxide from the bodies was also observed.
Throughout the extensive research undertaken over the past years by numerous investigators in seeking to form glass articles through the gelation of aqueous silicate solutions, the principal stumbling block to success was the development of cracking, flaking, and/or spalling brought on by capillary forces arising during drying of the gelled masses. Such forces were especially harmful where the masses contained very small pores, i.e., less than about 300A. Thus, a prime objective of all that research has been to develop porous silica masses wherein the pore diameters are such that only small capillary forces exist and where the latter forces are much weaker than the silica bond strengths.